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Chapter 12 | Thermodynamics 669
From Appendix G, here is the data:
Combining at 298 K:
Converting everything into kJ and combining at 298 K:
Substance
H2O(l)
−286.83
70.0
H2O(g)
−241.82
188.8
At 298 K (25 °C)
and so boiling is nonspontaneous (not spontaneous).
Check Your Learning
Use standard enthalpy and entropy data from Appendix G to calculate the standard free energy change for the reaction shown here (298 K). What does the computed value for ΔG° say about the spontaneity of this process?
Answer: the reaction is nonspontaneous (not spontaneous) at 25 °C.
The standard free energy change for a reaction may also be calculated from standard free energy of formation values of the reactants and products involved in the reaction. The standard free energy of formation is the free energy change that accompanies the formation of one mole of a substance from its elements in their standard states. Similar to the standard enthalpy of formation, is by definition zero for elemental substances under standard state conditions. The approach used to calculate for a reaction from values is the same as that
demonstrated previously for enthalpy and entropy changes. For the reaction
the standard free energy change at room temperature may be calculated as
Example 12.8
Calculation of
Consider the decomposition of yellow mercury(II) oxide.
